The effect of carbon content on the microstructure of hydrogen-free physical vapour deposited titanium carbide films

Titanium carbide (TiC) coatings for tribological applications were deposited on high speed steel. Several coatings with different titanium to carbon ratio were deposited by means of physical vapour deposition in which titanium was evaporated and carbon was sputtered. The coatings were characterised using analytical electron microscopy. It was observed that the change in titanium to carbon ratio significantly changed the microstructure of the coatings. The low carbon containing coatings consisted of columnar grains exhibiting a preferred crystallographic orientation whereas the coating with highest carbon content consisted of randomly ordered TiC grains in an amorphous carbon matrix. Energy filtered transmission electron microscopy revealed a change in Ti/C ratio as the distance from the substrate increased. The titanium to carbon ratio was observed to increase with distance from the substrate until a stable level was reached. This is due to a variation in the titanium evaporation during the early stages of film growth. This change of the titanium to carbon ratio affected the columnar growth in the initial stage of coating growth for the coatings with low carbon content.